The normal frequency range for human hearing is roughly 20 to 20,000 Hz. Vibration signals having frequencies of above about 20,000 Hz are in the ultrasonic range. Many industrial processes, including almost all sources of friction, create some ultrasonic vibrational noise. For example, leaks in pipes, machinery defects, and electrical arcing produce ultrasonic signals having frequencies too high for the human ear to detect.
In the past, ultrasonic sensors have been used in industrial settings to sense these ultrasonic signals and to detect abnormally high levels of ultrasonic sound which may indicate a mechanical fault in a machine. To monitor the ultrasonic signals produced by operating machinery, an operator would use an ultrasonic sensor to obtain a reading indicating the strength of the ultrasonic signals near the machine. If the ultrasonic signal levels generated by one machine were abnormally large, the operator would investigate further to determine if a problem existed with the noisy machine. If the ultrasonic signal levels were approximately equal to those produced by a properly functioning machine, the operator would assume the machine was properly functioning and simply proceed to the next machine.
Since ultrasonic signals generally cannot be heard by the human ear, ultrasonic sensing devices typically incorporate a heterodyne circuit to generate an audio signal that is a lower-frequency representation of the ultrasonic signal. The typical ultrasonic sensing device includes headphones or earphones to present the audio signal, which has a frequency within the range of human hearing, to the operator. Thus, when the ultrasonic sensing device detects an ultrasonic signal, the operator can hear an audio representation of the ultrasonic signal in the headphones.
As discussed above, machine elements such as bearings, valves, and gears, even those operating normally, produce vibration signals at both audible and ultrasonic frequencies. When trying to detect a machine fault using an ultrasonic sensing device, an operator listens for an abnormal signal, such as an impact sound, against a background of noise produced by the machinery. (For the purposes of this description, the term "background noise" is understood to include the noise that is present even in the absence of a fault condition, such as the noise generated by the normal operation of the machinery.) Many machine faults can be classified as impacts. For example, a bearing with a chip in a race, or a cracked gear tooth will produce an impact signal of only a few milliseconds or less in duration. The severity of the fault, the load conditions of the machine, as well as the amplitude of the normal background noise will determine the signal to noise ratio of the impact signal versus the background noise.
Given the noisy industrial environment and the brief, often transitory, nature of many fault signals that an operator is trying to hear, many important fault signals are not heard. To maximize the utility of the detector, it should be optimized to enhance the ability of the user to detect fault signals.